Exhaust gas recirculation is a technique commonly used for controlling the generation of undesirable pollutant gases and particulate matter in the operation of internal combustion engines. This technique has proven particularly useful in internal combustion engines used in motor vehicles such as passenger cars, light duty trucks, and other on-road motor equipment. The exhaust gas recirculation technique primarily involves the recirculation of exhaust gas by-products into the intake air supply of the internal combustion engine. The exhaust gas reintroduced to the intake manifold and subsequently the engine cylinders reduces the concentration of oxygen therein, which in turn lowers the maximum combustion temperature within the cylinder, decreasing the formation of nitrous oxide. Furthermore, the exhaust gases typically contain a portion of unburned hydrocarbon which is burned on its reintroduction into the engine cylinders, which further reduces the emission of exhaust gas by-products which would be emitted as undesirable pollutants from the internal combustion engine.
However, it is necessary to carefully control the proportion of recirculated exhaust gas to intake air. For example, while a greater proportion of exhaust gas may be recirculated at low load levels, it is necessary to ensure that the proportion of recirculated exhaust gas does not become excessive, causing the engine to stop due to a lack of sufficient oxygen to mix with the fuel so as to permit combustion. On the other hand, if the proportion of exhaust gas recirculated at full engine load is excessive, the power output of the internal combustion engine is reduced, and the engine will typically emit undesirable quantities of smoke and particulate matter due to unsatisfactory combustion in the engine cylinders. Therefore, it is clear that the exhaust gas recirculation process is desirably tightly controlled.
Another technique useful in the control and reduction of undesirable emissions from internal combustion engines is the use of pressure-charged intake air. This permits the use of relatively smaller cubic displacement and lighter weight internal combustion engines in mobile equipment, reducing in turn the specific fuel consumption of the vehicle and overall mass of the vehicle necessary to perform a given function. In addition to the benefits of reduced size and mass, the typical pressure-charging device may be controlled to provide improved emissions characteristics. Pressure-charging machines suitable for such applications include the exhaust gas driven turbocharger which is comprised typically of an exhaust gas driven turbine linked to a compressor disposed in the intake air stream to provide compression of the intake air. One way of controlling a turbocharger is to provide a gate which controls exhaust gas flow and gates exhaust gas to bypass the exhaust gas turbine and control the charging rate of the turbocharger so that the maximum pressure limits of the associated internal combustion engine are not exceeded. Another means of controlling a turbocharger is to provide a variable geometry turbocharger which allows for variation of the turbocharger vane position in response to engine speed or engine load or both. Change in vane position affects the manifold pressures within the engine air system which in turn affects the recirculation rate of exhaust gases from the exhaust manifold to the intake manifold.
Current EGR systems are generally used when an exhaust manifold pressure is greater than the pressure in the inlet manifold. In a pressure charged engine, including turbocharged and supercharged engines as examples, the pressure in the inlet manifold typically increases as the engine load increases. As the pressure in the exhaust manifold approaches the pressure in the inlet manifold, the exhaust gas recirculation flow in a fixed diameter orifice or duct between the inlet manifold and the exhaust manifold decreases. Higher engine speeds and engine loads also generally result in an increase in NOx emissions. Conventional EGR systems provide little, if any, exhaust gas recirculation during times when the engine is producing the most NOx, because the low pressure differential between the exhaust manifold and the inlet manifold prevents sufficient exhaust gas from entering the inlet manifold. Thus, EGR flow rate and turbocharger boost pressure represent engine operating parameters that are indirectly linked, yet often independently controlled. The present invention is aimed at overcoming the aforementioned problems.